Beyond the phenomenological macroscopic description of diffusion there are numerous
models exist to study the process.
For instance the inhomogeneity in a gas means a non-constant density, and usually also an inhomogeneous pressure.
General equations of motion of the one component fluid can be found in [1]}.
The case of binary diffusion means a diffusion of given particles among other ones which are different.
The binary diffusion has interesting properties and eventually spectacular in case if it is visible.
Regarding models of binary diffusion, it is worthwhile to mention the Lorentz gas model.
In this model certain results have been obtained related to diffusion by Machta and Zwanzig [2]},
Claus and Gaspard [3]}.
Connections with non-equilibrium thermodynamics were analyzed in [4]}, and [5]}.
Beyond diffusion Mátyás and Gaspard [6]} discussed diffusion with a simple reaction - isomerization -
where not only the diffusion coefficient, but the reaction rate is also evaluated.
One may also find diffusive processes where the diffusion is determined by the surroundings or the boundaries or the shape of the surface [7]}, [8]}, [9]}, [10]}.
